Photophysical and photocatalytic properties of Bi2MNbO7 (M = Al, In, Ga, Fe) thin films prepared by dip-coating

In this work we report the preparation and characterization of Bi2MNbO7 (M = Al, Ga, In, Fe) transparent thin films on glass slides. The films were obtained by dip-coating using bismuth(III) acetate, niobium(V) ethoxide and the corresponding metal(III) acetylacetonate precursors. Crystal structure and elemental analysis were performed by X-ray diffraction (XRD) and energy dispersive X-ray fluorescence (EDXRF). The band-gap energy (Eg) of the semiconductor films was estimated by UV–vis spectroscopy. Their photocatalytic activity was evaluated in the degradation of methyl orange (MeO) in aqueous solution. The presence of a crystalline phase in the Bi–M–Nb–O (M = Al, Ga, In, Fe) systems with pyrochlore-type structure was suggested by the diffraction peak at 2θ ≈ 29.01. The elemental proportion in the Bi–Ga–Nb–O film fits better to the stoichiometric ratio in Bi2MNbO7. The estimated Eg values were: Bi2FeNbO7 (2.47 eV) < Bi2GaNbO7 (2.67 eV) < Bi2AlNbO7 (2.79 eV) < Bi2InNbO7 (3.01 eV) and the calculated kinetic parameter t1/2 were: Bi2GaNbO7 (239 min) < Bi2AlNbO7 (278 min) < Bi2InNbO7 (296 min) < Bi2FeNbO7 (319 min). These results indicate that a slight band-gap narrowing has a positive effect in the photocatalytic properties but those further than Eg < 2.5 eV has a detrimental effect.